Thread: Bit operations - problem with rereading the data

I have a problem with bit operations. After packing a couple of numbers into one variable and trying to reread those values later, the output of the info function differs from the one of main function.

Code:

#include <iostream>using namespace std;


unsigned long long extract(unsigned long long value, int begin, int end)
{
    unsigned long long mask = (1 << (end - begin)) - 1;
    return (value >> begin) & mask;
}


unsigned long long encode(int caller, int caller_zone, int callee, int callee_zone, int duration, int tariff)
{
    unsigned long long encoded = 0;


    encoded |= caller;
    encoded <<= 17;


    encoded |= caller_zone;
    encoded <<= 7;


    encoded |= callee;
    encoded <<= 17;


    encoded |= callee_zone;
    encoded <<= 7;


    encoded |= duration;
    encoded <<= 13;


    encoded |= tariff;


    return encoded;
}


void info(unsigned long long u)
{
    int caller = extract(u, 47, 64);
    int caller_zone = extract(u, 40, 47);
    int callee = extract(u, 23, 40);
    int callee_zone = extract(u, 16, 23);
    int duration = extract(u, 3, 16);
    int tariff = extract(u, 0, 3);


    cout << "caller:        " << caller << endl
         << "caller_zone:   " << caller_zone << endl
         << "callee:        " << callee << endl
         << "callee_zone:   " << callee_zone << endl
         << "duration:      " << duration << endl
         << "tariff:        " << tariff << endl;
}


int main()
{
    int caller = 130999;    //17-bit number
    int caller_zone = 101;  //7-bit number
    int callee = 7777;      //17-bit number
    int callee_zone = 99;   //7-bit number
    int duration = 7000;    //13-bit number
    int tariff = 6;         //3-bit number


    cout << " FROM MAIN" << endl;
    cout << "caller:        " << caller << endl
         << "caller_zone:   " << caller_zone << endl
         << "callee:        " << callee << endl
         << "callee_zone:   " << callee_zone << endl
         << "duration:      " << duration << endl
         << "tariff:        " << tariff << endl;


    unsigned long long u = encode(caller, caller_zone, callee, callee_zone, duration, tariff);


    cout << "\n FROM INFO" <<endl;
    info(u);
}

Do you know what am I missing here?

With all those magic numbers and strange things going on I gave up trying to understand and just said "Hodor!" (and, no, Hodor is not scared of bit/bitwise operations and/or manipulation)

I don't know if you'd be interested, but C++ provides a built-in method for just that, namely bit fields. For example:

Code:

struct myBitField
{
    unsigned long long caller : 17;
    unsigned long long caller_zone : 7;
    unsigned long long callee : 17;
    unsigned long long callee_zone : 7;
    unsigned long long duration : 13;
    unsigned long long tariff : 3;
};

You can use the members as plain struct ones, and the whole struct has "sizeof(unsigned long long)". If you want to manipulate the whole value, you can create a union, like so:

Code:

union myBundle
{
    myBitField bitField;
    unsigned long long llvalue;
};